1. Field: This invention relates to magnesium oxide compositions, and particularly to magnesium oxide compositions which have been modified to improve strength and sinterability.
2. Prior Art: The production of ceramic bodies from magnesium oxide has long been practiced. Generally, sintering of magnesium oxide ceramic bodies has been done either by hot-pressing or by hot isostatic pressing. Both of these techniques require the application of pressure at elevated temperatures to achieve the appropriate densification of the magnesium oxide material into a relatively strong ceramic body. Even under pressurized sintering conditions, magnesium oxide has generally not possessed the strength of other well-known ceramic materials, such as alumina, silicon carbide, silicon nitride, and the like.
Ceramic bodies of magnesium oxide have generally been employed in instances where particular attributes of magnesium oxide were useful even though such bodies were not especially strong. Such attributes include a low thermal conductivity, high coefficient of thermal expansion, and high melting point. Such uses have not been fully exploited due to the somewhat lower overall strength and fracture toughness of magnesium oxide bodies including ones which have been prepared by pressurized sintering.
Attempts to sinter magnesium oxide bodies in the absence of pressure have generally required fairly high sintering temperatures, that is, sintering temperatures significantly above 1400.degree. C. Sintering temperatures in this range tend to promote grain growth of crystals within the ceramic particles. This generally tends to diminish the overall strength and fracture toughness of the resulting ceramic body.
One of the advantages of pressurized sintering is that some of the compaction which is desired during sintering is accomplished by the high pressure involved. Thus, during pressurized sintering, lower sintering temperatures may be employed for short periods of time. This is desirable since deleterious grain growth is a function of residence time and temperature.
Techniques for improving the strength of many types of ceramics, such as alumina, beta-alumina, silicon nitride and the like have involved the inclusion of zirconium oxide which transforms from a tetragonal crystal state to a monoclinic crystal state to provide a toughened article. U.S. patents relating to such compositions include: U.S. Pat. Nos. 4,298,385 and 4,322,249 of Claussen, et al.; 4,184,882 and 4,187,116 of Lange; and 4,218,253 of Dwarok, et al.
In an article published in 1985, subsequent to the filing date of October 1984 of the parent application, Ser. No. 665,912, authors Ikuma, et al. discuss transformation toughening of ceramics generally and, in particular, certain of their work which included magnesium oxide bodies containing zirconium oxide; "ZrO.sub.2 -Toughened MgO and Critical Factors in Toughening Ceramic materials by Incorporating Zirconia," Journal of Materials Science Letters 4 (1985) 63-66.